With the increased emphasis on the control of air and water pollution, mechanical methods of sludge dewatering have become an essential part of water treatment processes. No longer can untreated sludge simply be dumped into the nearest river, lagoon or vacant lot. With this increased environmental concern, sludge concentrating, and dewatering techniques have been given ever increasing emphasis in industry.
Sludge dewatering processes, which include vacuum filtration, centrifugation, lagoon and gravity cell dewatering, date back to the early 1800's. Sludge dewatering is not a simple, quick process. In sludge handling facilities, problems encountered in the dewatering process include high moisture cake formation, poor cake release from dewatering equipment, high disposal costs, slow dewatering and poor equipment efficiency.
Sludge dewatering can lead to increased savings, especially with respect to transportation costs associated with sludge disposal. Even small decreases in the sludge moisture content can result in significant economic benefits.
Sludge is almost always given primary dewatering treatment before discharge from any given process system. Primary dewatering is usually accomplished using thickener/clarifiers or settling ponds. Secondary dewatering, including vacuum filtration, centrifugation, belt filters, lagoons, etc., are then commonly employed to further increase the solids content, so that the resulting sludge comprises only about 40-80% liquid.
In certain instances, sludge conditioning agents, commonly polyelectrolytes, may be added to the sludge to aid in the production of a drier cake, and to reduce wear and tear on dewatering equipment.
U.S. Pat. No. 3,414,514 (Buhl) discloses sludge dewatering processes comprising contacting the sludge with acrylamide-beta methacrylyloxyethyltrimethylammonium methyl sulfate (MTMMS) copolymer. It is stated therein that the copolymer consists by weight of 99%-20% acrylamide and 1-80% MTMMS. Preferred monomer ratios given in the patent are 97%-50% acrylamide and 30-50% MTMMS.
In U.S. Pat. No. 4,152,307 (Shibahara et al), certain water in oil polymer emulsions are disclosed as being "useful in waste water treatments". Specifically, the emulsions are formed by dispersing from 30-70% by weight of an aqueous solution of an ethylenically unsaturated (hydrophilic) monomer and 70 to 30% by weight of a hydrophobic organic liquid in the presence of a surfactant, and then polymerizing the resultant dispersion to obtain a suitable water in oil emulsion. The ethylenically unsaturated monomer may be used in conjunction with another hydrophilic monomer. Included within the scope of this broad disclosure, as being useful hydrophilic monomers, are, inter alia, 2-(meth)acryloyloxyethyltrimethyl ammonium chloride [which may also be referred to as dimethylaminoethyl methacrylate methylchloride] and acrylamide.
Other patents which may be of interest include U.S. Pat. Nos. 3,336,269 (Monagle et al) and 3,336,270 (Monagle). These disclosures pertain, inter alia, to preparatory routes for acrylamide type water soluble polymers in general and detail the preparation of acrylamide/vinyl quaternary ammonium salt copolymers, such as beta-methacryloyloxyethyltrimethyl ammonium methyl sulfate/acrylamide copolymers. Another patent related to the general field of flocculation is U.S. Pat. No. 3,278,506 (Chamot et al).
In contrast to the prior art disclosures, I have found that a very high cationic charge density copolymer is surprisingly effective as a biosludge dewatering agent. Specifically, my invention pertains to a copolymer having about a 80-90% cationic charge density. The cationic monomer is a quaternary ammonium salt of 2-(meth)acryloyloxyethyl-N,N-dimethylamine. This specific cationic monomer is copolymerized with acrylamide in a molar ratio of about 80-90% cationic monomer: 20-10% acrylamide.